The human eye is an incredibly detailed system that processes incoming light signals and adjusts to constantly changing circumstances. The eye has the ability to adapt to different lighting conditions that may originate a variety of distances from the eye, and then convert this light to electrical impulses that are transmitted to the brain to produce the images that people see.

Major Structures and Image Formation

Crucial components of the eye that allow humans to process images are the cornea, pupil, lens, and retina. These parts are somewhat analogous to the parts of a camera. The cornea is like a lens cover, and its job is to receive light and focus it through the pupil, which is analogous to the camera's aperture. From there, the light is focused on the back of the eye at the retina, which is somewhat like camera film. Special photoreceptor cells in the retina then convert light signals to electrical impulses, which are transmitted to the brain via the optic nerve.

Eye Sensitivity and Color Perception

The special photoreceptor cells that transmit signals to the brain are comprised of 6 to 7 million cones and about 120 million rods. These cells work together, day and night, to process images and transmit them to the brain. However, rod and cone cells each predominate under different lighting conditions.

Cones are the cells that allow people to perceive color, and each cone is classified according to three different types of color reception: green, red, and blue. Cones are less sensitive to light than rods and are the primary mechanism for daytime vision.

Rods far outnumber cones, and are up to one thousand times more sensitive to light than cones, but do not perceive color. Rods are responsible for night vision, also called photoscopic vision. Rods take much longer to adapt to changing light conditions, which is why it can take up to 30 minutes to fully adjust to nighttime vision when entering a dark room.

The following page contains links to resources on the most common vision problems: